The present invention relates to the fabrication of silicon carbide films, particularly to pulsed energy synthesis and doping of beta silicon carbide, and more particular to a method for fabricating and doping beta silicon carbide thin films using pulsed laser processing.
Silicon carbide is an attractive material for numerous applications, and its high bandgap, high thermal conductivity, and high electron saturation velocity make it attractive for high power electronic device applications. Also, its hardness and transparency make it attractive for than film optical coatings. Silicon carbide (SIC) can exist in 170 polytypes, most of which are hexagonal or rhombohedral .alpha.-SiC, and .alpha.-SiC and coatings thereof have been produced by various techniques and for various applications. The prior techniques for preparing ceramic coatings including .alpha.-SiC coated substrates, which include the use of laser energy are exemplified by U.S. Pat. Nos. 4,426,405 issued Jan. 17, 1984 to F. J. Hierholzer, Jr. et al.; 4,446,169 issued May 1, 1984 to P. M. Castle et al.; 4,476,150 issued Oct. 9, 1984 to D. N. Rose; 4,534,997 issued Aug. 13, 1985 to G. R. Brotz; 4,925,830 issued May 15, 1990 to F. Walsh; and 5,151,966 issued Sep. 29, 1992 to C. Brehm et al.
While SiC can exist in numerous polytypes, only one type of cubic polytype, or cubic SiC has been identified, and denoted as 3C-or .beta.-SiC. Thin films of beta silicon carbide (.beta.-SiC) are of great interest since the electron transport properties are much better than .alpha.-SiC over a wide temperature range of 27.degree.-730.degree. C. Epitaxial films of .beta.-SiC have been primarily grown by chemical vapor deposition, with significant doping problems. Chemical vapor deposition of high purity .beta.-SiC on the surface of a substrate is exemplified by U.S. Pat. No. 5,106,687 issued Apr. 21, 1992 to K. Tanino et al. By way of example, heterojunction bipolar transistor (HBT) devices have been fabricated using epitaxial .beta.-SiC emitters on Si substrates, and high DC HFE gain of 800 was observed with undetectable recombination current, see T. Sugii et al., IEEE Elec. Dev. Lett 9(2) 87 (1988). HBT devices have also been fabricated using polycrystalline .alpha.-SiC, but the current gain was limited by interface recombination which may have resulted from the low deposition temperature (600.degree. -900.degree. C.) required to prevent dopant redistribution in the silicon.
Deposition methods for silicon carbide include electron-beam-heated evaporation, plasma-enhanced chemical vapor deposition, vacuum sublimation, rapid thermal chemical vapor deposition, and chemical vapor deposition, including the use of off-axis substrates. The crystallinity resulting from these techniques is best near or above 1000.degree. C., and the heterointerface quality appears to decrease below this temperature as well. The high temperatures required in these conventional processes can destroy the underlying device structures.
It has been determined by the present inventors that laser processing of thin films of amorphous silicon and carbon co-deposited on various substrates produces .beta.-SiC, with very small crystallites, tens of angstroms in size, but no other phases (Si, C, or SiC) were found. Also, it has been determined that doped .beta.-SiC may be produced by introducing dopant gases during the laser processing. Thus, the present invention fills a need for a reliable, controllable, low cost method for producing thin films of .beta.-SiC and for doping it, at low substrate temperatures.